1. Field of the Invention
The present invention relates to devices used to measure current flowing through high voltage electrical power transmission lines.
2. Related Art
The measurement of high currents present in power transmission lines has previously been accomplished with devices which use iron-core transformers. The winding ratio is chosen so that the large primary current is converted to a proportional smaller secondary current. The use of iron-core transformers has a number of disadvantages including the weight of such transformers and the difficulty involved, and time required, in installation. A further important disadvantage is that iron-core transformers have an inherent saturation point. At the saturation point, the magnetic flux density in the iron core does not maintain a linear relationship with the primary current, and, as a result, the secondary current is not proportional to the primary current. An accurate measurement of the primary current cannot be accomplished when the secondary current is not proportional to the primary current.
In order to avoid the limitations associated with iron-core transformers, some devices have used air-core transducers, such as Rogowski coils. One particular advantage of air-core transducers is the elimination of saturation problems. Saturation can not occur in air-core transducers. The physics of air-core transducers are such that the output of the coil is a voltage which is proportional to the time derivative of the primary current. As a result, one limitation of conventional devices which incorporate air-core transducers is that an integration operation must be performed before the magnitude and phase of the primary current can be determined. The integration operation has been performed in two locations.
Any electronics mounted electrically close to the power transmission line (the source end) requires a power source. When the integration operation is performed at the source end, additional power is required to power the integration circuits. Another approach has been to perform the integration operation at a location where the metering equipment is located (the remote end) to eliminate the additional power required for the integration operation at the source end. However, when the integration operation is performed at the remote end, the link connecting the source end and the remote end must have a very wide frequency bandwidth and a large voltage dynamic range. One skilled in the art will appreciate that equipment having a wide bandwidth and large dynamic range is inherently more susceptible to electrical noise effects than other equipment.
In accordance with the invention, an air-core current transducer measurement system is provided which overcomes the problems associated with both iron-core current transformers and conventional air-core current transducers and which combines the advantages associated with conventional air-core current transducers (e.g., ease of installation, no saturation under overload, and small size and weight) with the permanent installation advantages of iron-core current transformers. The measurement system of the invention is particularly useful, inter alia, in providing alternating current measurements at specific locations in a power system and, more generally, in connection with metering and protective systems for electrical power equipment.
According to the invention, an air-core current transducer measurement system is is provided for measuring current flow in a high voltage electrical conductor such as transmission line, the system comprising: a high voltage end including an electrically conductive case for shielding the electrical circuitry at that end and an isolated low voltage end coupled to the high voltage end and providing a scaled output representing the current flow in the high voltage electrical conductor. A current transducer located at the high voltage end includes a coil for sensing current flow in the conductor or transmission line and produces a time derivative output signal. An electrostatic shield connected to the conductive case provides electrostatic shielding of the coil. A low pass filter, connected to the coil and to the input of electrically conductive case, filters out electrical noise. An integrator, located in the conductive case, converts the time derivative signal into a scaled signal corresponding to the scaled output signal provided at the low voltage end.
Preferably, the high voltage end and low voltage end are connected together by a fiber optic cable. Advantageously, the conductive case further includes a signal converter for converting the signal produced by the integrator into a light signal for transmission over the fiber optic cable. The air-core transducer measurement system preferably further comprises a light signal converter at said low-voltage end for converting the light signal into the scaled output signal.
In an advantageous implementation, a laser is located at the low voltage end, a photovoltaic power supply is located at the high voltage end for supplying electrical energy to the integrator and the signal converter, and a fiber optic cable couples the laser to the photovoltaic power supply. A laser power safety interlock circuit is preferably provided for removing power from the laser when a predetermined fault condition occurs. Advantageously, the interlock circuit receives as an input the light signal produced by the signal converter.
The current transducer preferably comprises a Rogowski coil, a torroidal coil with a core material with a relative permeability of approximately unity. The integrator typically comprises an analog integrator, although the integrator can also be implemented digitally.
Other features and advantages of the invention will be set forth in, or apparent from, the following detailed description of the preferred embodiments of the invention.